This invention is a process for producing a quality halftone copy from a halftone original and more specifically is a method for enhancing the edges in the halftone copy.
It is customary in the printing industry to produce halftone copies from gray scale continuous tone originals. However, in convenience copying and high volume reproduction, it frequently is necessary to reproduce a halftone copy from a halftone original. If the halftone original is screened by using the same screening process used for continuous tone originals, the dot pattern of the original will interact with the screening process to form regular noise patterns, Moire patterns, which are obvious and distracting.
It is better to separate the continuous tone and halftone originals and use a different process for each. The most obvious way to handle a halftone original is to scan the original, filter out the screen pattern by low pass filtering, and then rescreen the image. Of course, edge detail in the original is lost in the process of filtering out the original screen pattern.
U.S. Pat. No. 4,259,694, "Electronic Rescreen Technique for Halftone Pictures", by Henry Liao and Hsieh S. Hou, describes a method which rescreens the original halftone without destroying the edge detail. This involves a two path system. In one path, a traditional low pass filter removes the screen, along with the path, a traditional low pass filter removes the screen, along with the detail. In the other path, an edge extraction process finds and enhances the edges. Finally, the results of both paths are added to produce a continuous tone image with good edge detail, which is then halftoned in the usual way. The edge extraction process described herein is intended to operate in such a system and therefore that patent is incorporated herein by reference.
In general, it is easier to detect an edge, and to compute the direction and magnitude of an enhancement for it, if the bit position in question is clearly on one side of an edge or the other. To use an example, if the bit in question, the current bit, is a black bit and if all bits in the same row and all rows beneath it are also black, and if all bits in the rows above are white, then clearly the current bit is on the black side of a horizontal edge. On the other hand, it should be equally clear that if the edge is in the form of a curve, or if there is a significant noise content in the bit pattern, then for a current bit on the edge, there is a probability of some magnitude that a needed enhancement will not be made, or will be made in the wrong direction.